What nitrix says is right, though his instructions may be too brief to be enough for you to figure it out.
I've been meaning to put up some video demos and tutorials but i've been really sick for the last couple of days and so i haven't been so productive lately.
To do what you want there is 1 mandatory step, and one optional step.
The mandatory step is to go create an entry in the Search Folders tab with the directory you want to sometimes search. For that directory, specify an Optional Modifier keyword, like: tv
Once you do that, the tv directory will be searched (and only be searched) when you add +tv to your search, for example: "family guy +tv"
Now the second step comes in if you dont want to have to type +tv, you can do what nitrix says and create an alias to simulate this, by making an alias with a regular expression like ^tvshow (.*) and for the result of that specify dosearch +tv $$1
The creation of the alias is a bit contrived in this case since it doesnt save you much typing, but there are cases where it might be useful.